The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Light emitting diode (LED)-based illumination systems are being increasingly used particularly in commercial applications. Some examples of commercial applications where LED-based illumination systems are used include billboards, computer displays, and television screens. LED-based lamps can also be used in home and office environments. For example, LED-based lamps having the shape of a conventional light bulb or a tube light can be used in home and office environments. LED-based lamps that can be used in home and office environments, however, are not yet as affordable as incandescent and fluorescent lamps.
Lamps that generate white light are generally preferred in home and office environments. LEDs can be used to manufacture lamps that generate white light. For example, LEDs that generate red, green, and blue light can be used to manufacture lamps that generate white light. Specifically, light generated by red, green, and blue LEDs can be combined to produce white light. LEDs that generate pure red and green light, however, can be relatively expensive.
Alternatively, LEDs that generate blue light and phosphors that convert blue light into red and green light can be used to produce white light. Specifically, blue LEDs can be coated with a mixture of red and green phosphors. Some of the blue light output by the blue LEDs is converted to red and green light by the red and green phosphors, respectively. Some of the blue light output by the blue LEDs may escape the phosphors without getting converted. The red and green light converted by the phosphors combines with the blue light that escapes unconverted to produce white light.
The mixture of red and green phosphors produces optimum light output when excited by blue light having specific wavelengths. For example, most red and green phosphors convert blue light optimally when the wavelength of the blue light is approximately 450 nm. Accordingly, blue LEDs that produce blue light within a narrow range of wavelengths (e.g., 450 nm±5 nm) are typically selected to generate white light, and blue LEDs that produce light having wavelengths outside of the narrow range of wavelengths are typically rejected. The stringent selection process and rejection of numerous LEDs increases the cost of generating white light using blue LEDs. Additionally, the coating of the phosphor mixture may not be uniform across the LEDs. Due to variations in the coating, the whiteness of the light produced by the LEDs may vary from LED to LED. Accordingly, the LEDs need to be selected using a binning process, which further increases cost.